Conventionally, the following fuel cell system is known (for example, refer to Patent Literature 1): At a system stop, the fuel cell system continues the fuel gas supply to an anode side of the fuel cell and meanwhile stops the oxidant gas supply to the cathode side of the fuel cell. In such a state, the fuel cell system extracts the current from the fuel cell and consumes the cathode side oxygen of the fuel cell, so as to prevent deterioration of the fuel cell.
In the fuel cell system described in the Patent Literature 1; the voltage of the fuel cell during the time when the current is being extracted from the fuel cell is monitored, and the current extraction from the fuel cell is ended at the timing when the fuel cell voltage is decreased to such a value that the cathode side oxygen is determined to be sufficiently consumed.